The present invention is directed towards concrete compositions and methods for the preparation thereof. More specifically, the invention is directed in its preferred embodiments towards xe2x80x9clightweightxe2x80x9d or low-density concrete compositions that generally comprise a cementitious material and an aggregate material that is relatively less dense than aggregate materials used in conventional concretes.
Concrete is a hardened material that is formed by blending a cementitious material, typically Portland cement, with an aggregate material and water to form an uncured concrete composition, and subsequently curing the concrete composition. Conventionally used aggregate materials include gravel, shale, stone, and like materials. Concretes that are formulated with such conventional aggregate materials have a density that typically ranges from about 140 to 150 lbs./cu. ft. Such concrete is suitable for use in connection with many applications, and such concretes are particularly suitable for use in connection with load-bearing structural applications.
The prior art has recognized that in many applications xe2x80x9clightweightxe2x80x9d concretes are desirable. xe2x80x9cLightweightxe2x80x9d concrete, or low-density concrete, may be defined generally as a concrete with a density of less than the 140-150 lb./cu. ft. of conventional concretes. Such lightweight concretes often have a density of less than about 120 lbs./cu. ft. Lightweight concrete may be used in many applications including, for example, concrete construction blocks; insulating materials; decorative applications such as yard ornaments, patio or walkway blocks, and the like; upper-floor construction in multi-storied buildings; maritime applications, such as docks, floating bridges, boats, aircraft carrier decks, off-shore oil platforms, and the like; floors in steel frame buildings; roof structures or in roofing tiles; wallboard; spray-on applications; insulating and fire-proofing liners; backfill; and in numerous other applications.
The prior art has taught to prepare lightweight concrete using lightweight aggregates, including such materials such as perlite, vermiculite, polystyrene beads, polyethylene beads, pumice, scoria, tuff, or expanded materials such as shale, clay, slate, diatomite, fly ash, dross, or blast furnish slag. Many such aggregates are expensive and of limited availability. For example, polystyrene beads are petroleum-based and are in limited supply. Cinders generated in coal-fired boilers have been employed as aggregate materials, but increasing environmental awareness of the adverse impact of coal-fired boilers suggests a long-term supply problem. Similarly, materials such as pumice or scoria may require substantial labor or fuel costs or expensive and cumbersome processing equipment. Dross, a by-product of the iron and steel industries, can be expanded to form a lightweight aggregate by placing it into contact with water. However, dross is not uniform in its properties and thus may be difficult to process under exacting quality control conditions.
The prior art has further taught to prepare lightweight concretes by employing air voids in the concrete to reduce the overall concrete density. Generally, the prior art teaches to provide such air voids by entraining air in the concrete mix to form xe2x80x9ccellsxe2x80x9d in the concrete matrix. While such methods generally will result in an overall decrease in cured concrete density, entraining air to form such cells often is less than a satisfactory solution. Air bubbles can compress at the bottom of deep concrete pours resulting in smaller cells than desired, thus resulting in a concrete of variable internal density and higher weight than desired. Air bubbles can also coalesce to form larger bubbles, thus resulting in a concrete of variable internal density and relatively reduced strength in the region of the air bubble.
More recently, U.S. Pat. No. 5,595,595 purports to teach the use of xe2x80x9caquagels,xe2x80x9d or starch pastes, as an aggregate material in a concrete composition. According to the teachings of this patent, aggregate material is prepared by cooking starch until it gelatinizes, spraying this gelatinized starch into a water-immiscible liquid, washing the aquagel thus formed to remove the water-immiscible liquid, and storing the starch until it is ready to be incorporated into a concrete composition. Alternatively, the gelatinized starch may be retrograded, dried, and ground into appropriately sized particles. When preparing a concrete composition, the dried particles are hydrated to thereby swell the particles to cause an increase in the volume. Upon curing of the concrete composition, water is said to be liberated from the swollen particles, thus causing them to shrink to their unhydrated size thereby providing a plurality of voids, or xe2x80x9ccells,xe2x80x9d in the concrete.
The concrete compositions and aggregate materials postulated by the aforementioned U.S. Pat. No. 5,595,595 are unsatisfactory in a number of respects. For one thing, the preparation of the aquagel material in the manner taught is time-consuming, cumbersome, and expensive. In addition, the patent teaches that the aquagels preferably are chilled after washing to prevent bacterial growth and degradation, thus potentially entailing additional processing costs. Moreover, the dried and ground particles must be pre-hydrated in situ prior to adding to the other components of the concrete composition. The need for such in situ hydration is inconvenient and unwieldy, and potentially limits the applicability of the concrete composition.
The present invention is based upon the discovery that a concrete composition and, particularly, if desired, a lightweight concrete composition, can be prepared using water, a cementitious material, and an aggregate material that comprises granular starch, and more preferably, washed, pre-swollen crosslinked granular starch. Starch is an abundant and renewable natural resource, and is substantially inexpensive relative to other lightweight aggregate materials provided in the prior art. Moreover, especially when the starch is provided in the form of washed, pre-swollen crosslinked granules, there is no need to pre-hydrate the starch granules prior to mixing with the other components of the concrete composition. The invention thus can provide an inexpensive, lightweight composition that incorporates as an aggregate material an abundant, renewable resource.
In accordance with one embodiment of the invention, a curable composition comprising water, a cementitious material, and an aggregate material that includes a plurality of swollen starch granules is provided. It is believed that the starch granules will liberate water upon curing of the concrete composition, thus causing the volume of the granules to decrease. This deswelling provides a cured concrete composition that includes a plurality of cells similar to those formed by entraining air in the concrete, but of uniform size and dispersion in the concrete matrix. Thus, the invention also provides a cured concrete composition that comprises water, a cementitious material, and an aggregate that comprises a plurality of water-liberated swollen starch granules. The cured concrete composition preferably is a lightweight concrete composition, i.e., it has a cured bulk density of less than 120 lbs./cu. ft., more preferably, less than about 115 lb./cu. ft. The invention further encompasses a method for preparing a concrete composition, as well as a concrete composition prepared in accordance with this method.
Preferred features and other embodiments of the invention are set forth hereinbelow and in the appended claims.
The concrete composition of the invention generally comprises water, a cementitious material, and an aggregate material that, generally speaking, comprises granular starch. The water provided in the composition may be fresh water, tap water, sea water, natural or artificial brine, or, in general, any water source as may be determined suitable for use in a given application. The water is present in the uncured concrete composition in an amount effective to provide a curable concrete composition. Other additives as are known or as may be discussed to be useful in connection with concrete may be employed, and it is presently contemplated that set retarders or accelerators, water reducers, superplasticizers, air entraining chemicals, damp-proofers, colorants, corrosion inhibitors, and other additives may be used to impart desirable characteristics or working properties.
The cementitious material may be any suitable cementitious material known in the art or otherwise found to be suitable. While the preferred cementitious materials are selected from the Portland cements, other cementitious materials may be used, and among those known in the art, it is contemplated that rapid-setting cements, alumina cement, alumina/calcium oxide and/or calcium hydroxide cement, JET cement (consisting of a mixture of calcium homo-aluminate and calcium sulfate), Portland white cement, high-alumina cement, natural cements, gypsum cements and plasters, calcium aluminate cement, magnesia cements, common lime, hydraulic lime, puzzolan, and in general any other cementitious material that is suitable for fabrication into a concrete composition may be useful in conjunction with the invention.
The concrete composition further includes an aggregate material. In accordance with the invention, the aggregate material comprises plural starch granules. The granular starch used in conjunction with the invention may be any suitable granular starch, such as corn starch, high-amylose starches, wheat starch, potato starch, rice starch, tapioca starch and, in general, any starch that is compatible with the cementitious material. The starch preferably is present in a ratio of about 1% to about 20%, more preferably, from about 3% to about 10% dry basis weight by dry basis weight of the cementitious material, i.e., the total dry formula weight (the starch weight being calculated on an unhydrated basis), but more generally can be present in any other amount relative to cementitious material that is suitable to provide aggregation in the cured concrete. The starch should not be used in an amount that would prevent the concrete from setting (curing). Generally, concrete is considered to have set when a compression strength of 4000 psi is achieved, as measured, for example, by ASTM C403, although it is contemplated that in some applications a lower cured compression strength may be acceptable.
The starch is present in the uncured concrete composition in the form of a plurality of swollen starch granules. By xe2x80x9cswollenxe2x80x9d is contemplated granules that are hydrated with an amount of water effective to increase the granule size relative to the dry granule size. It is believed that in most or all embodiments of the concrete composition of the invention the starch granules will liberate water upon curing of the concrete composition and the volume of the starch granules will therefore decrease, thus creating cells in the cured concrete. While it is not intended to limit the invention to a particular theory of operation, the step of curing the concrete generally contemplates hydration of the cementitious material possibly accompanied by loss of water due to evaporation and/or bulk liquid drainage. These phenomena are believed to cause the liberation of water from the starch granules to form water-liberated starch granules that are believed to be of a relatively smaller size than the swollen starch granules. The starch granules generally have a granular size ranging from about 5 microns to about 30 microns nominal diameter when unswollen, and up to about 150% this diameter when swollen. It is believed that the starch granules may liberate water such that the granules may have a final moisture content greater than 0%, but less than the moisture content of the swollen starch granules in the uncured concrete composition.
In preferred embodiments of the invention, the starch granules comprise crosslinked starch granules. Numerous methods for crosslinking starch granules are known in the art, and in conjunction with the invention, any one of numerous crosslinking agents, such as sodium trimetaphosphate, epichlorohydrin, phosphorous oxichloride, dimethyl urea, or other chemicals known to crosslink starch or otherwise may be found in crosslinked starch may be used. The amount of crosslinker used likely will vary between different starches but, in general, should be used in an amount such as to allow the starch granules to swell when heated in water, but to maintain their swollen state without dissolving or pasting out. It is believed that extensive gelatinization or pasting of the starch will retard the curing of the concrete. For example, when epichlorohydrin is used as a crosslinking agent, it can be used in an amount ranging from about 0.01% to 1% by dry basis weight of the starch being crosslinked, more preferably, about 0.1% to about 0.5%. When sodium trimetaphosphate is used, it preferably is used from an amount ranging from about 0.5 to 2.5% by dry basis weight of starch. One suitable commercially available crosslinked starch is sold by Grain Processing Corporation of Muscatine, Iowa under the trademark PURE-DENT(copyright) B850, which is a crosslinked dent corn starch. Other suitable starches include PURE-GEL(copyright) B990, B992, B994, B996, and B980 also sold by Grain Processing Corporation of Muscatine, Iowa.
The extent of cross-linking of the starch granules will determine the level of swellability and of soluble organic content. Low levels of cross-linking will allow great swelling, which will result in a lower density cured concrete. However, the low levels of cross-linking will result in more soluble organic material, limiting the amount of starch which can be added and still allow the composition to set. Higher levels of cross-linking may limit or eliminate the ability of the starch granule to swell. The extent of cross-linking of the starch generally should not be such as to cause the concrete to fail to set while allowing the starch granules to be swellable.
Crosslinked starch granules when swollen in hot water include up to about 1% water-soluble organic material. As little as 0.005% of such organic material per dry basis weight of cementitious material in the dry concrete formula may undesirably or unacceptably retard the curing of the concrete. Thus, in accordance with the preferred embodiments of the invention, the starch granules are washed to substantially remove the soluble organic materials. In a highly preferred embodiment of the invention, the starch granules are washed and pre-swollen with water to reduce the amount of soluble organic materials to less than about 0.05% soluble organic material by dry basis weight of the entire starch granule. This washed starch then is dried.to reduce the moisture content of the starch below about 12%, and preferably in the range of about 9% to about 12% moisture. Starch granules that have been pre-swollen, washed and dried in such manner are approximately the same size as original starch granules; such starch granules may be referred to as RS (or rapid-swelling) starch granules. Such RS starch granules will quickly imbibe water and swell to their original swollen size upon being placed in contact with water. It is contemplated that by washing the starch granules, more starch may be incorporated into a concrete composition than by using unwashed starch granules, thereby allowing for preparation of a relatively lighter weight concrete at comparable aggregate loading levels.
Upon adding the starch granules to the water and cementitious material, the concrete is molded or formed into the desired shape or configuration and is allowed to cure. The curing of the uncured concrete composition may be performed via any suitable method, such as by heating, but, in general, the curing may be accomplished simply by allowing the concrete to set under ambient conditions. For example, the concrete composition can be poured into a substantially void mold or form and allowed to harden.
Once cured, the concrete composition can be made to have a density of less than about 130 lbs./cu. ft., more preferably, a density of less than about 120 lbs./cu. ft., more preferably, a density of less than about 115 lbs./cu ft., and even more preferably, a density ranging from about 90 to about 100 lbs./cu. ft. However, while the starch-based concrete compositions of the invention are suitable for use in connection with the preparation of such lightweight concretes, the invention is not limited thereto, and indeed starch may be used in connection with other aggregate materials in a concrete composition other than a lightweight concrete composition. If a lightweight concrete is desired, the starch may be used in connection with other lightweight concrete aggregates or entrained air in a lightweight concrete composition. Thus, the starch may be used, for example, in connection with a conventional aggregate material such as gravel or a conventional lightweight aggregate material such as pumice or others as previously mentioned. In addition, the starch may be used in relatively small amounts to provide a concrete composition having a density comparable to regular concrete composition, for example, a density of about 140 lbs./cu. ft. or greater.
The invention also encompasses a method for preparing a concrete composition. In accordance with this embodiment of the invention, the method comprises the steps of mixing a water, a cementitious material, and an aggregate to form an uncured composition, the aggregate comprising a plurality of starch granules as set forth above, and curing the uncured concrete composition. The starch granules may be added to the other components of the uncured concrete composition in swollen form or in dry form. When added in dry form, the amount of water used in the composition should generally be adjusted to account for the amount of water needed to swell the starch granules, generally increased by about one to five times the weight of the starch added. The invention further encompasses a concrete composition prepared in accordance with the method of the invention.
In carrying out the method or composition of the invention, various conditions of temperature, pH, and the like may be employed. The temperature and pH of the composition generally will be at ambient conditions, but both will rise during the curing process as the water reacts with the cement portion of the composition. No special condition of temperature or pH are contemplated. Other conditions as may conventional or otherwise found to be suitable may be employed.